It is promoted that an alcohol-containing fuel obtained by mixing gasoline with alcohol, which is a biofuel extracted, for instance, from sugarcane, corn, or wood, be used as an automotive fuel. Under such a circumstance, flexible fuel vehicles (FFVs), which can use various types of fuels that differ in alcohol concentration (alcohol content), have been progressively studied for development purposes.
Gasoline and alcohol differ in stoichiometric air-fuel ratio. The stoichiometric air-fuel ratio for gasoline is approximately 14.6, whereas the stoichiometric air-fuel ratio, for instance, for ethanol is approximately 9. Therefore, the stoichiometric air-fuel ratio for an alcohol-containing fuel varies with its alcohol concentration. Consequently, when the employed fuel is changed to a fuel having a different alcohol concentration, it is necessary to change the air-fuel ratio accordingly.
In an internal combustion engine, feedback control is generally exercised over the air-fuel ratio in accordance with a signal output from an exhaust gas sensor that generates an output according to the air-fuel ratio of exhaust gas. While such air-fuel ratio feedback control is exercised, no problem arises even if the employed fuel is changed to a fuel having a different alcohol concentration, that is, a different stoichiometric air-fuel ratio. The reason is that the amount of fuel injection is automatically corrected so as to equalize the exhaust air-fuel ratio with the stoichiometric air-fuel ratio.
However, air-fuel ratio feedback control is stopped during fuel increase. Catalyst protection fuel increase is performed to prevent an exhaust purification catalyst from overheating. Power fuel increase is performed to generate a higher power. If the employed fuel is changed while air-fuel ratio feedback control is stopped, the air-fuel ratio difference brought about by the fuel change will not be fed back. Therefore, the fuel injection amount cannot be corrected. This may degrade emissions and driveability. In addition, the following problem may also arise.
Catalyst protection fuel increase is a correction that is made by increasing the fuel injection amount to provide an air-fuel ratio lower than the stoichiometric air-fuel ratio for the purpose of lowering the exhaust temperature by using fuel vaporization heat when the catalyst is likely to overheat. Let us now assume that the currently employed fuel injection amount is calculated to provide an air-fuel ratio of 12 in a situation where catalyst protection fuel increase is performed during an operation performed through the use of a fuel composed of 100% gasoline. Let us also assume that the employed fuel is changed to a fuel having a high alcohol concentration (e.g., a fuel having an alcohol concentration of 85%) while the catalyst protection fuel increase is performed. For a fuel having a high alcohol concentration, an air-fuel ratio of 12 is leaner than the stoichiometric air-fuel ratio. In this case, therefore, the effect of exhaust temperature decrease by the fuel vaporization heat lessens, thereby allowing the exhaust temperature to rise. As a result, the catalyst may become damaged, and in the worst case, may melt down.
Meanwhile, an air-fuel ratio control apparatus disclosed in JP-A-5-5446 stores in advance learned values for air-fuel ratio correction, sorts the stored learned values by alcohol concentration, allows an alcohol concentration sensor installed in a fuel tank to detect the alcohol concentration of fuel, and selectively uses a learned value in accordance with the alcohol concentration of fuel fed.    Patent Document 1: JP-A-5-5446    Patent Document 2: JP-A-2005-98265    Patent Document 3: JP-A-2005-90427    Patent Document 4: JP-A-9-324693